Originally Posted by BlueB
I just measured/calculated the frame area, it's about 14 sqin. So, that theory is pretty much out of the window...
You are right. It's surprising how persistent the "Head measures from the outside of the frame" myth is. I guess if you hear a thing often enough it starts to seem undeniably true.
Here is the only plausible explanation I have heard: At one point Head rounded off their frame head size designations to 600, 630, and 660 cm^2. It was for marketing purposes; 577, 613, and 660 aren't as easy to get customers to remember. Head maintains these designations to this day, presumably to keep some consistency from one line of rackets to the next.
This was not an uncommon practice. The Prince "Series 110" rackets, for example, ranged from ~102 sq" to ~108 sq", while the "Series 90" rackets (that I know of) had 93 sq" heads. It is only in the last decade or two that head size more precise than "oversize", "midplus", "mid", and "standard" even mattered to consumers.